We are familiar with the reversible reaction between weakly basic anion resin with strongly acidic ions. The reaction equations are as follows:
Absorption:
2R--N+2H.sup.+ +SO.sub.4.sup.-2.fwdarw.(R--NH).sub.2 SO.sub.4 PA1 R--N+H.sup.+ +Cl.sup.-.fwdarw.R--NH.cndot.Cl.sup.- PA1 (R--NH).sub.2 SO.sub.4 +2OH.sup.-.fwdarw.2R--N+SO.sub.4.sup.-2 +2H.sub.2 O PA1 R--NH.cndot.Cl.sup.- +OH.sup.-.fwdarw.R--N+Cl.sup.- +H.sub.2 O PA1 46 g(formic acid equivalent gram).times.1.25eq/L(resin absorption capability)=57.5g/L PA1 57.5 g/L.times.80%.times.6000L/R.div.25 ppm=11040 m.sup.3 /cycle PA1 14.3 g/L.times.6000 L/R.div.25 ppm=3432 m.sup.3 /cycle PA1 3432 m.sup.3 /cycle.div.11040 m.sup.3 /cycle.times.100%=31% PA1 Theory Formic Acid Absorption Capacity: PA1 The efficiency of Actual acid elimination: PA1 C.sub.2 H.sub.4 +3O.sub.2.fwdarw.2CO.sub.2 +2H.sub.2 O PA1 C.sub.2 H.sub.4 +O.sub.2.fwdarw.2HCHO (Formaldehyde) PA1 HCHO+1/2O.sub.2.fwdarw.HCOOH (Formic Acid) ##STR1##
Regeneration:
According to these equations, we know that the main function of weakly basic anion resin is to absorb strongly ionic acids, such as H.sub.2 SO.sub.4, HCl, HNO.sub.3, etc. But for weak acid (for example, formic acid and acetic acid), the absorption become worse. If the water contain both strong acid and weak acid at the same time, the weakly basic anion resin will absorb strong acid firstly, then weakly acid. This method has some problems: The theory formic acid absorption capability for the general resin provided by vendor is considered with 80% efficiency. Because the real plant can not follow pH and absorption curve to treat the process water, the target value can not be made.
A real plant has a column with 6000-liter weakly basic anion resin to treat process water whose formic acid concentration is 25 ppm. From periodic water adoption, we know that:
(1.) The theory formic acid absorption capability for weakly basic anion resin PA0 (2.) If the efficiency is considered as 80% for the theory formic acid absorption capability of weakly basic anion resin, we can get the theory periodic water adoption as follow. PA0 (3.) After implementing an examination for the formic acid absorption capability, we get the periodic water adoption as follow. PA0 (4.) Theory vs. exam. PA0 (5.) From the data described above. If we do not follow pH and absorption curve to treat the process water, the product rate is merely 31%. PA0 1. Main reaction: C.sub.2 H.sub.4 +1/2O.sub.2.fwdarw.C.sub.2 H.sub.4 O (EO) PA0 2. Side reaction: PA0 3. Esterification--Reaction of Formic or Acetic Acid with Ethylene Glycol ##STR2##
Note:
Formic acid equivalent gram multiply acid absorption equivalent gram per liter resin (refer to the character of resin), represent as g/L. PA2 Actual Formic Acid Absorption Capacity: PA2 Amount of water adoption.times.ion concentration.div.amount of resin.div.1000, represented as g/L PA2 Actual Formic Acid Absorption capacity through examination.div.Theory Formic Acid Absorption capacity.times.100% represented as %.
The concentration of formic acid is direct relative to pH value, and pH value will influence absorption capacity of weakly basic anion resin. Because of ion absorbability reducing, formic acid will be leaked before reaching resin fully saturated condition, and elimination rate will be influenced. Using weakly basic anion resin to treat weak acid, the elimination will not complete and it will be disadvantaged to recover wastewater.
This invention tries to treat some weak acid such as fornic acid and acetic acid. In ethylene glycol process, if formic acid and acetic acid in the process water did not eliminate well and we reuse this recovery water, this will reduce ethylene glycol product rate and cause corrosion problems to equipment and pipes. Therefore it is important to eliminate acid well and provide good quality recovery water.
In ethylene glycol (EG) process, organic acids (formic acid and acetic acid) comes from the side reaction of ethylene oxide (EO) reaction. EO and EG reaction mechanism are described as follows:
In ethylene glycol process, we use circulation water to scrub ethylene oxide, also formic acid and acetic acid at the same time. Ethylene oxide reacts with water and then produces ethylene glycol. If water contain formic acid and acetic acid, it will esterificate with ethylene glycol and cause corrosion problems to equipment and pipes. Therefore formic acid and acetic acid must be eliminated.